BioMEMS devices are microelectromechanical systems which have a sensing element for biological applications, and often have an optical or electrical signal conduit that connects between a packaged fluidic device and the outside environment. A cap enclosing the packaged fluidic device from the outside environment is frequently used, with the cap bonded directly over the optical signal conduits as they run to fluidic material retained by the cap. Traditionally, adhesive bonding with polydimethylsiloxane (“PDMS”) or anodic bonding might be used to bond the cap over the signal conduit, but this requires the conduit introduce a minimal topography change (for example, <1000 angstroms in the case of anodic bonding). Use of PDMS as the bonding agent, bonding directly on the conduit, has been commonly used, as the index of refraction is similar to SiO2 and therefore does not interfere with optical signal conduit operation. Generally, this BioMEMS system using PDMS is easy to prototype and does not require a cleanroom environment. However, in some cases PDMS introduces conduit topography contribution limitations that generally require the signal conduit to be less than 200 nanometers high because the PDMS cannot securely bond over a greater topography variation. Additionally, PDMS absorbs molecules such as fluorophores in capped areas critical for signal generation, and, in certain situations, may be too porous, allowing water to evaporate thought the seal.
Additionally, anodic bonding, or bonding directly on the conduit via an electrostatic field, requires high voltage, particularly for a conduit thicker than a few hundred angstroms, typically requiring around 800V-1000V to achieve a proper bond. The high energy associated with such anodic bonding can change the physical properties of the optical or electrical conduit, making it less efficient or otherwise unsuitable. Additionally, anodic bonding suffers from conduit topography contribution limitation (<100 nm) as well, and sometimes requires a temperature up to about 400 to 500 degrees Celsius, which may make it incompatible for BioMEMS device fabrication on a CMOS wafer.
Other methods used in the adhesive bonding technique of a fluidic device include using glue or epoxy to bond a polycarbonate cap directly on to the conduit. However, these materials used in such a technique may interfere with the function of the signal conduit.